On *' Intertraction " between Albuminous Substances, etc. 119 



lighter salt solutions upon heavier albuminous fluids — adding generally to one 

 or other fluid a trace of colouring matter (eosin) in order to render the course 

 of events more manifest to the eye. The experiments were conducted in 

 capillary tubes, full sized test-tubes and also in a very convenient form of 

 diffusion cell suggested by my colleague, Dr. Alexander Fleming. The 

 diffusion cell just referred to is made by covering a microscopic slide, or 

 larger sheet of glass with a layer of wax of any desired thickness ; cutting out 

 a cell of any desired shape ; and then bringing down upon the border of wax 

 a companion microscopic slide or glass plate. 



The phenomena described below manifest themselves alike in each of the 

 above mentioned types of receptacle. For demonstration and final experi- 

 mentation the flat cell has of course obvious advantages. When in such a 

 cell serum is allowed to run down gently from a pipette on to the surface of 

 a heavier — e.g., 6 percent. — saline solution the following train of events occurs. 



When the sermn impinges upon the surface of the salt solution it indents 

 it, and then takes up -by hydrostatic resilience a position on the surface, 

 floating there as a layer which is delimited below by a somewhat undulate 

 outline. Then within a very few seconds — seemingly as a result of whirlpool 

 movements sucking in the wave summits protruding downwards from the 

 under surface of the serum — this specificially lighter fluid is drawn down into 

 the heavier salt solution below. The appearance is then as if pseudopodia or 

 tentacles were being let down into the depths (figs. 1 and 2). Simultaneously 

 with this, as can be seen when we employ a coloured salt solution and an 

 uncoloured serum, the former is carried up into the serum forming there a 

 system of thin ascending streams arranged after the fashion of the teeth of 

 a comb (fig. 2). 



This down- and up-streaming process progresses apace and gives, as an 

 intermediate result : a stratum of transported serum upon the floor of the cell ; 

 and a layer of transported salt solution ranged at the top of the cell super- 

 ficially to the original stratum of serum. As a terminal result, we have 

 complete interfusion, manifested to the eye by quite uniform coloration. 

 With a difl'usion cell made from microscopic slides (i.e., a cell measuring 

 1 inch by 3 inches) this is arrived at in something like half-an-hour. 



It will be seen that we have here two arresting features : the singular 

 fashion in which the lighter and heavier fluids interpenetrate (we may perhaps 

 speak of this as " pseudopodial mterpenetration "), a,nd the rapidity with which 

 complete interfusion is achieved. 



The singular point about the pseudopodial interpenetration is not so much 

 that a lighter fluid (the serum) is carried down into a heavier one ; but that 

 this serum, instead of recoiling to the top, sinks to the bottom, like a heavier 



L 2 



